Former Secretary of State Hillary Clinton. Thomson Reuters Democratic presidential candidates are descending on Iowa for the first face-off of their 2016 primary, a contest that remains dominated by the outsized political influence of Hillary Rodham Clinton.

All five Democratic primary candidates are on the program for a dinnertime fundraiser sponsored by the state party in Cedar Rapids, creating an opportunity for her challengers to confront Clinton before more than 1,200 influential party activists in the crucial caucus state.

Three months into what seems like an all-but-inexorable march to the nomination, Clinton has already built a vast campaign infrastructure, establishing a multistory headquarters in Brooklyn, New York, and placing hundreds of staffers across the country.

But her first joint event with her primary rivals comes amid signs that she has yet to win over her party's most passionate supporters, the activists and small-dollar donors that will form the base of her support in the general election.

At a town-hall meeting in New Hampshire on Thursday, liberal environmental protestors broke out into chants after Clinton refused to promise an immediate halt to all fossil-fuel development.

"I totally respect the passion and the urgency," she said, attempting to calm the crowd. "I understand it."

An Associated Press-GfK poll released this week found her standing falling among Democrats, with about 70% of Democrats giving Clinton positive marks, an 11-point drop from an April survey. Nearly one-quarter of Democrats now say they see Clinton in an unfavorable light.

"I don't like seeing that, obviously," Clinton said of the poll, speaking to reporters on Thursday. "But I think people know that I will fight for them. I'll fight for their jobs. I'll fight for their families. I'll fight on behalf of better education and health care."

She added: "I'm very pleased with the support I have."

Former Secretary of State Hillary Clinton. Ethan Miller/Getty Images

Just 17% of the $47 million that Clinton raised since announcing her campaign came from contributions of $200 or less. In comparison, Vermont Sen. Bernie Sanders has fueled his insurgent challenge to Clinton with small donations, pulling in three-quarters of his more than $15.2 million haul from smaller amounts.

In recent weeks, Sanders has packed arenas with voters eager to hear the message of the self-described socialist, who's become Clinton's chief rival. So far, he's refused to directly criticize Clinton, though he's questioned her positions on issues like trade, Wall Street regulations, and the Keystone XL pipeline.

"I like her. I respect her," Sanders said on Tuesday, after joining his fellow Senate Democrats at a luncheon with Clinton on Capitol Hill. "It is not necessary for people to dislike each other or attack each other just because they're running for office."

Besides Sanders and Clinton, the forum includes former Maryland Gov. Martin O'Malley, former Virginia Sen. Jim Webb, and former Rhode Island Gov. Lincoln Chafee. Each candidate will deliver 15 minutes of remarks.

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